Conditional immortalization of human B cells by CD40 ligation

It is generally assumed that human differentiated cells have a
limited life-span and proliferation capacity in vivo, and that
genetic modifications are a prerequisite for their immortalization
in vitro. Here we readdress this issue, studying the long-term
proliferation potential of human B cells. It was shown earlier that
human B cells from peripheral blood of healthy donors can be
efficiently induced to proliferate for up to ten weeks in vitro by
stimulating their receptor CD40 in the presence of interleukin-4.
When we applied the same stimuli under conditions of modified cell
number and culture size, we were surprised to find that our
treatment induced B cells to proliferate throughout an observation
period of presently up to 1650 days, representing more than 370
population doublings, which suggested that these B cells were
immortalized in vitro. Long-term CD40-stimulated B cell cultures
could be established from most healthy adult human donors. These B
cells had a constant phenotype, were free from Epstein-Barr virus,
and remained dependent on CD40 ligation. They had constitutive
telomerase activity and stabilized telomere length. Moreover, they
were susceptible to activation by Toll-like receptor 9 ligands, and
could be used to expand antigen-specific cytotoxic T cells in
vitro. Our results indicate that human somatic cells can evade
senescence and be conditionally immortalized by external
stimulation only, without a requirement for genetic manipulation or
oncoviral infection. Conditionally immortalized human B cells are a
new tool for immunotherapy and studies of B cell oncogenesis,
activation, and function.